It looks like this method in Springboard does some date/time calculations to determine if the current build is about to expire, every time the cover sheet gets dismissed. For some reason, the latest builds of iOS 12 think they’re about to expire.

ZDNet is able to confirm that this update does indeed fix the problem. And ahead of the Labor Day weekend.

Fortunately, the update is pretty small, coming in at under 100MB, so it shouldn’t take too long to download and install.